The invention relates to a laser layout with an optical resonator cavity bounded by a fully reflecting mirror and a partially reflecting mirror, in which an active laser medium and a crystal for the generation of second and higher harmonics are located.
A laser layout of this type with a Q switch to control the pulsed operation of the laser is known for example from U.S. Pat. No. 4,617,666 and contains a so-called intracavity-SHG-crystal for the doubling of frequency on the basis of the effect exhibited by many crystals of producing a second harmonic (second harmonic generation). The doubling of the light frequency in lasers is a relatively simple process to enlarge the spectrum of the frequencies available. This takes place in particular in the case of lasers, the capacities of which are within the range of 80 and 100 Watts, and which are used for example for the engraving of plastic or metal surfaces. It is frequently desirable here to obtain high output capacities, with power densities in the resonator approaching the destruction threshold of the SHG crystals on their antireflection coatings. This, however, easily leads to the destruction of the expensive SHG crystals, if lasers in industrial use cannot be maintained by competent personnel, and rough environmental conditions, in particular thermal variations in the laser configuration, alter the setting of the components of the laser, so that excessive power densities occur in the resonator, which overload the SHG crystal.
In U.S. Pat. No. 4,181,899 a laser for the generation of second harmonics in crystal is described, the fundamental wavelength of which amounts to 1.06 .mu.m. The generation of harmonics is limited by the heating of the crystal by thermo-optical effects, which leads to erroneous phase matching and which reduces the conversion efficiency. This limitation is overcome in the known laser by the tuning of the laser wavelength, thereby obtaining an improved conversion efficiency of the generation of the harmonics. The tuning is effected by determining the crystal temperature of the known laser equipped with a Q switch. By means of a dielectric polarizer, part of the frequency-doubled outlet light is intercepted at a wavelength of 0.53 .mu.m on a light detector, which makes it possible to detect the intensity of the frequency-doubled output light, in order to tune by means of a variable operating voltage, a tuning element.
A laser oscillator with an output modulator according to Swiss patent publication No. 544,426 comprises a photodetector, which detects the light exiting from the resonator with a transmission of 0.1% of the resonator, the capacity of which is proportional to the optical power density in the laser resonator. The optical resonator has as a modulatable output mirror a tunable interferometer. The known layout measures the power of the laser light by means of a photodetector during the tuning of the interferometer, wherein the proportional electrical signal may be passed to a peak value detector and a comparator.
A laser beam monitoring system according to U.S. Pat. No. 4,544,839 contains a transparent zinc selenide disk provided in the external beam path of the laser, which permits the passage of about 75% of the energy of the laser beam of a CO.sub.2 laser. The partial beam intercepted at an inclination of 83.degree. impacts through a focusing lens a photosensitive sensor. The sensor is connected with an amplifier, the outlet of which controls a shutter diaphragm of the laser.
From French patent publication No. 2,171,866 a process and an apparatus are known for the stabilization of the energy of a laser beam, which also make it possible to interrupt the emission of the laser beam, if it exceeds a predetermined intensity. The known apparatus comprises an intercepting mirror for part of the laser light output which impacts a photoelectric cell and actuates, by means of a comparator, a power amplifier to which a pumping light source is connected. By comparing the electrical signal with a reference signal, it is possible to interrupt the operation of the pumping light source if a predetermined power is exceeded.